The human macrophage line U937 and the human T-lymphoblastoid line HSB2 have been rendered sensitive to hypoxanthine-aminopterin-thymidine (HAT) culture medium by treatment with 8-azaguanine. The HSB2 cell line was fused to T lymphocytes that were stimulated with concanavalin A for 48\hours. Hybridomas were generated that constitutively produced interleukin 2 and chemotactic factor. Eight of these hybridomas have been cloned and continue to constitutively secrete (Greater than 12 months) interleukin 2 in concentrations from 2 to 40 times that of an equal number of mitogen-stimulated peripheral blood lymphocytes. One clone also produces monocyte chemotactic factor and other lines produce fibroblast activating factor. Fusion of the U937 line with purified human monocytes has not led to successful clones. However, fusion of human monocytes to the murine myeloma line 653 led to stable human-mouse hybridomas with twice the DNA content of the parent 653 line. The characteristics of these hybridomas and their secretory products are currently being investigated.